Ink jet printing is a non-impact method that in response to a digital signal produces droplets of ink that are deposited on a substrate such as paper or transparent film. Ink jet printers, especially thermal or bubble jet drop-on-demand printers have found broad application as output for personal computers in the office and the home.
Thermal ink jet printers use a battery of nozzles each containing a resistor element to fire ink droplets toward the print media. Nozzle openings are typically about 40-60 micrometers in diameter. These small dimensions require inks that do not plug these small openings.
Therefore, a critical requirement for an ink jet ink is the ability to remain in a fluid condition in a pen opening on exposure to air, so called "decap" conditions. This allows a pen to function after a period of non-use or during operation of infrequently utilized nozzles. A major concern with all ink jet printers is pluggage of nozzles during operation and between operations. This is caused by evaporation of an organic solvent or water from the surface of the nozzle. In dye-based inks, this can cause crystallization or precipitation of soluble components such as dye or solid additives. In pigment-based inks this evaporation can cause:
a. precipitation of the dispersion PA1 b. flocculation of the pigment dispersion PA1 c. precipitation of solid additives. PA1 (a) an aqueous carrier medium, PA1 (b) a pigment dispersion or dye; and PA1 (c) a nitrogen heterocyclic diol having a solubility in water of at least 4.5 parts in 100 parts of water at 25.degree. C, and which is the reaction product of a heterocyclic compound which is monocyclic and comprises either a 5- or 6-membered ring which comprises 2 trivalent nitrogen atoms with one carbon atom spaced between them and wherein the nitrogen atoms are usually adjacent to 1-3 amide carbonyl groups and said carbon atom is a part of one carbonyl group; and an alkylene oxide selected from the group consisting of ethylene oxide, propylene oxide, and mixtures thereof. PA1 B is a divalent group completing a 5- or 6-membered ring and is selected from the group consisting of: ##STR2## wherein R=--H, --CH.sub.3, --C.sub.2 H.sub.5, or --C.sub.3 H.sub.8 ##STR3## wherein R and R' =--H, --CH.sub.3, --C.sub.2 H.sub.5, or --C.sub.3 H.sub.8 ##STR4## wherein R=--H, --CH.sub.3, --C.sub.2 H.sub.5, or --C.sub.3 H.sub.8 ##STR5## wherein R and R'=--H, --CH.sub.3, --C.sub.2 H.sub.5, or --C.sub.3 H.sub.8, ##STR6##
Initial evaporation generally causes an increase in viscosity which affects the ability of the nozzle to fire a drop of ink since ink jet pens are designed to operate within specific viscosity ranges. The inception of pluggage may cause distortion of the image or alphanumeric character. This may appear as a drop of ink which is displaced from its intended position. Sometimes two drops will be formed equally spaced from the intended target position. Sometimes small numerous satellite drops are produced. On some occasions the drop may even reach its intended position but at a lower drop volume producing a lower optical density image. Ultimately the plugged nozzle will fail to fire and no image will be generated.
Ink jet printers are designed to prevent excessive evaporation of solvent from pen nozzles by seating the pen cartridge in an air tight chamber when not in use. These devices become ineffective with continued printer use because dried ink deposits at the rubber seals and the system loses its air-tight condition. Another device used is a wiper that removes solid formed at the surface of the nozzle. This device is often ineffective because of the depth of the plug or because of sufficient hardness of the plug which thereby resisting mechanical removal. Another pluggage fix is the use of forced air or vacuum suction to clear the nozzle. These devices are often inefficient and add considerable expense to the cost of the printer.
A commonly used scheme to cure pluggage is to clear the plug by firing the pen in a non-image mode, i.e., into a collection receptacle or "spittoon". While this solution is the most effective remedy, it requires that the ink form a "soft" or noncohesive plug. To make this non-image pluggage clearance effective the surface plug must be mechanically or cohesively weak.
The compounds of this invention produce plugs that are easily removed by non-image firing or spitting.
A critical measured property for an ink jet ink is the "decap time" which is the length of time over which an ink remains fluid in a pen opening when exposed to air and therefore capable of firing a drop of ink at its intended target. "Decap" means the pen is uncovered or "decapped". The "decap" test is run by firing all pen nozzles successively 100 times at several programmed incrementally increasing time intervals. Each ink is given a time score for the first, 5th and 32nd drop firing. This time interval is the longest interval that the pen fires a specified drop without drop displacement or loss of density. The longer the time rating the more desirable the ink. Results are reported in Table 2.
The ability of a plug to be removable by non-image firing is measured by 5th and 32nd drop decap time values. It is highly desirable to obtain a long time interval for the fifth drop because this means the plug is easily removed with only 4 non-image firings. An ink with long decap values for both 5th and 32nd drop indicates a soft plug forms and is readily cleared.
A second important requirement for ink jet inks where the colorant is a pigment is for the pigment dispersion to remain stable throughout the life of the ink jet cartridge. Dye-based ink jet inks suffer from deficiencies in water fastness, smear fastness and lightfastness. Therefore, pigments are a preferred alternative to dyes provided the pigment dispersions can be made stable to flocculation and settling. Some cosolvents that are good pluggage inhibitors or promote soft plugs cause destabilization of pigment dispersions and therefore cannot be used in pigmented inks, e.g., Butyl Carbitol.RTM. and Dowanol TBH.RTM.. (See Comparative Examples 2 and 3, Table 2.)
Accordingly, a need exists for cosolvents which function as pluggage "softeners" or inhibitors without destabilizing pigment-based inks.